Coolant comprising azole derivatives for cooling systems in fuel-cell drives

ABSTRACT

Methods for cooling a fuel cell include providing in a fuel cell coolant system an aqueous fuel cell coolant composition having an initial electrical conductivity of at most 50 μS/cm. The fuel cell coolant compositions include an alkylene glycol and water solution, and an additive consisting of one or more five membered heterocyclic compounds (azole derivatives) having 2 or 3 heteroatoms selected from the group consisting of nitrogen and sulfur, which contain no or at most one sulfur atom and which may carry a fused aromatic or saturated six-membered ring, and an orthosilicate in an amount to provide a silicon content of from 2 to 2000 ppm by weight.

This application is a divisional of U.S. patent application Ser. No.10/477,463 filed on Nov. 12, 2003 now U.S. Pat. No. 7,371,334, which inturn is the U.S. national phase of international applicationPCT/EP/02/06194 filed Jun. 6, 2002 which designated the United States,the entire content of each prior filed application being expresslyincorporated hereinto by reference.

The present invention relates to coolants for cooling systems infuel-cell drives, in particular for motor vehicles, based on alkyleneglycols or derivatives thereof, which comprise specific azolederivatives as corrosion inhibitors.

Fuel cells for mobile use in motor vehicles have to be capable ofoperation even at low outside temperatures of down to about −40° C. Afreezing-protected coolant circuit is therefore vital.

The use of the conventional radiator protection compositions employed ininternal-combustion engines would be impossible in fuel cells withoutcomplete electrical insulation of the cooling channels, since thesecompositions, owing to the salts and ionizable compounds present thereinas corrosion inhibitors, have high electrical conductivity, which wouldadversely affect the function of the fuel cell.

DE-A 198 02 490 (1) describes fuel cells having a freezing-protectedcooling circuit in which the coolant used is a paraffinic isomer mixturehaving a pour point of below −40° C. However, the combustibility of acoolant of this type is disadvantageous.

EP-A 1 009 050 (2) discloses a fuel-cell system for automobiles in whichthe cooling medium used is air. However, it is disadvantageous here thatair, as is known, is a poorer thermal conductor than a liquid coolingmedium.

WO 00/17951 (3) describes a cooling system for fuel cells in which thecoolant employed is a pure monoethylene glycol/water mixture in theratio 1:1 without additives. Since, owing to the lack of corrosioninhibitors, absolutely no corrosion protection would be present againstthe metals present in the cooling system, the cooling circuit containsan ion exchanger unit in order to maintain the purity of the coolant andto ensure a low specific conductivity for an extended time, preventingshort-circuits and corrosion. Suitable ion exchangers mentioned areanionic resins, for example of the strongly alkaline hydroxyl type, andcationic resins, for example based on sulfonic acid groups, and otherfiltration units, for example activated carbon filters.

The construction and mode of functioning of a fuel cell for automobiles,in particular a fuel cell having an electron-conducting electrolytemembrane (“PEM fuel cell”, “polymer electrolyte membrane fuel cell”) isdescribed by way of example in (3), the preferred metal component in thecooling circuit (radiator) being aluminum.

DE-A 100 63 951 (4) describes coolants for cooling systems in fuel-celldrives which comprise orthosilicates as corrosion inhibitors.

The use of azole derivatives, such as benzimidazole, benzotriazole ortolutriazole, as corrosion inhibitors in radiation protectioncompositions for conventional internal-combustion engines operated withgasoline or diesel fuel has been known for some time, for example from:G. Reinhard et al., “Aktiver Korrosionsschutz in wäBrigen Medien”, pp.87-98, expert-Verlag 1995 (ISBN 3-8169-1265-6).

The use of azole derivatives of this type in coolants for coolingsystems in fuel-cell drives has not been disclosed hitherto.

The main problem in cooling systems in fuel-cell drives is themaintenance of a low electrical conductivity of the coolant in order toensure safe and fault-free functioning of the fuel cell and to preventshort circuits and corrosion in the long term.

Surprisingly, it has now been found that the time duration for lowelectrical conductivity in a cooling system based on alkyleneglycol/water, also and in particular if it contains an integrated ionexchanger in accordance with (3), can be significantly extended by theaddition of small amounts of azole derivatives. This offers thepractical advantage that the time intervals between two coolant changesin fuel-cell drives can be extended further, which is of particularinterest in the automobile sector.

Accordingly, we have found antifreeze concentrates for cooling systemsin fuel-cell drives which give ready-to-use aqueous coolant compositionshaving a conductivity of at most 50 μS/cm, based on alkylene glycols orderivatives thereof, which comprise one or more five-memberedheterocyclic compounds (azole derivatives) having 2 or 3 heteroatomsfrom the group consisting of nitrogen and sulfur, which contain no or atmost one sulfur atom and which may carry a fused aromatic or saturatedsix-membered ring. Preference is given here to antifreeze concentrateswhich comprise a total of from 0.05 to 5% by weight, in particular from0.075 to 2.5% by weight, especially from 0.1 to 1% by weight, of saidazole derivatives.

These five-membered heterocyclic compounds (azole derivatives) usuallycontain, as heteroatoms, two N atoms and no S atom, 3 N atoms and no Satom or one N atom and one S atom.

Preferred groups of said azole derivatives are fused imidazoles andfused 1,2,3-triazoles of the general formula (I) or (II)

where the variable R is hydrogen or a C₁- to C₁₀-alkyl radical, inparticular methyl or ethyl, and the variable X is a nitrogen atom or theC—H group. Typical examples of azole derivatives of the general formula(I) are benzimidazole (X=C—H, R=H), benzotriazole (X=N, R=H) andtolutriazole (tolyltriazole) (X=N, R=CH₃). A typical example of an azolederivative of the general formula (II) is hydrogenated1,2,3-tolutriazole (tolyltriazole) (X=N, R=CH₃).

A further preferred group of said azole derivatives comprisesbenzothiazoles of the general formula (III)

where the variable R is as defined above, and the variable R′ ishydrogen, a C₁- to C₁₀-alkyl radical, in particular methyl or ethyl, orin particular the mercapto group (—SH). A typical example of an azolederivative of the general formula (III) is 2-mercaptobenzothiazole.

Preference is furthermore given to non-fused azole derivatives of thegeneral formula (IV)

where the variables X and Y together are two nitrogen atoms or onenitrogen atom and one C—H group, for example 1H-1,2,4-triazole (X=Y=N)or imidazole (X=N, Y=C—H).

Very particularly preferred azole derivatives for the present inventionare benzimidazole, benzotriazole, tolutriazole, hydrogenatedtolutriazole or mixtures thereof.

Said azole derivatives are commercially available or can be prepared bycommon methods. Hydrogenated benzotriazoles and hydrogenatedtolutriazole are likewise accessible in accordance with DE-A 1 948 794(5) and are also commercially available.

Besides said azole derivatives, the antifreeze concentrates according tothe invention preferably additionally comprise orthosilicates, asdescribed in (4). Typical examples of orthosilicates of this type aretetraalkoxysilanes, such as tetraethoxysilane. Preference is given hereto antifreeze concentrates, in particular those having a total contentof from 0.05 to 5% by weight of said azole derivatives, which giveready-to-use aqueous coolant compositions having a silicon content offrom 2 to 2000 ppm by weight of silicon, in particular from 25 to 500ppm by weight of silicon.

Dilution of the antifreeze concentrates according to the invention withion-free water gives ready-to-use aqueous coolant compositions having aconductivity of at most 50 μS/cm and which essentially consist of

-   (a) from 10 to 90% by weight of alkylene glycols or derivatives    thereof,-   (b) from 90 to 10% by weight of water,-   (c) from 0.005 to 5% by weight, in particular from 0.0075 to 2.5% by    weight, especially from 0.01 to 1% by weight, of said azole    derivatives, and-   (d) if desired orthosilicates.

The sum of all components here is 100% by weight.

The present invention thus also relates to ready-to-use aqueous coolantcompositions for cooling systems in fuel-cell drives which essentiallyconsist of

-   (a) from 10 to 90% by weight of alkylene glycols or derivatives    thereof,-   (b) from 90 to 10% by weight of water,-   (c) from 0.005 to 5% by weight, in particular from 0.0075 to 2.5% by    weight, especially from 0.01 to 1% by weight, of said azole    derivatives, and-   (d) if desired orthosilicates.    and which are obtainable by dilution of said antifreeze concentrates    with ion-free water. The sum of all components here is 100% by    weight.

The ready-to-use aqueous coolant compositions according to the inventionhave an initial electrical conductivity of at most 50 μS/cm, inparticular 25 μS/cm, preferably 10 μS/cm, especially 5 μS/cm. Theconductivity is kept at this low level in long-term operation of thefuel-cell drive over a number of weeks or months, in particular if acooling system with integrated ion exchanger is used in the fuel-celldrive.

The pH of the ready-to-use aqueous coolant compositions according to theinvention drops significantly more slowly over the operating time thanin the case of cooling fluids to which said azole derivatives have notbeen added. The pH is usually in the range from 4.5 to 7 in the case offresh coolant compositions according to the invention and usually dropsto 3.5 in long-term operation. The ion-free water used for the dilutionmay be pure distilled or bidistilled water or water that has beendeionized by, for example, ion exchange.

The preferred mixing ratio by weight between the alkylene glycol orderivatives thereof and water in the ready-to-use aqueous coolantcompositions is from 20:80 to 80:20, in particular from 25:75 to 75:25,preferably from 65:35 to 35:65, especially from 60:40 to 40:60. Thealkylene glycol component or derivatives thereof which can be used hereis, in particular, monoethylene glycol, but also monopropylene glycol,polyglycols, glycol ethers or glycerol, in each case alone or in theform of mixtures thereof. Particular preference is given to monoethyleneglycol alone or mixtures of monoethylene glycol as the principalcomponent, i.e. with a content in the mixture of greater than 50% byweight, in particular greater than 80% by weight, especially greaterthan 95% by weight, with other alkylene glycols or derivatives ofalkylene glycols.

The antifreeze concentrates according to the invention which give theready-to-use aqueous coolant compositions described can themselves beprepared by dissolving said azole derivatives in alkylene glycols orderivatives thereof which are water-free or have a low water content(for example up to 10% by weight, in particular up to 5% by weight).

The present invention also relates to the use of five-memberedheterocyclic compounds (azole derivatives) having 2 or 3 hetero atomsfrom the group consisting of nitrogen and sulfur, which contain no or atmost one sulfur atom and which may carry a fused aromatic or saturatedsix-membered ring for the preparation of antifreeze concentrates forcooling systems in fuel-cell drives, in particular for motor vehicles,based on alkylene glycols and derivatives thereof.

The present invention furthermore relates to the use of these antifreezeconcentrates for the preparation of ready-to-use aqueous coolantcompositions having a conductivity of at most 50 μS/cm for coolingsystems in fuel-cell drives, in particular for motor vehicles.

The coolant compositions according to the invention may also be employedin a fuel-cell unit as described in DE-A 101 04 771 (6), in which thecooling medium is additionally electrochemically deionized in order toprevent corrosion.

EXAMPLES

The invention is explained in the following examples, but without beingrestricted thereto.

In the test described below, the coolant compositions according to theinvention were tested for their suitability for fuel-cell drives incomparison with a coolant composition as described in (3):

Description of the Experiment:

Five aluminum test metals (vacuum-sold-red Al, name: EN-AW 3005,solder-plated on one side with 10% by weight of EN-AW 4045; dimensions:58×26×0.35 mm with a hole having a diameter of 7 mm) were weighed,connected in a non-conductive manner by means of a plastic screw withwasher and Teflon disks and placed on two Teflon stands in a 1 l beakerwith ground-glass joint and glass lid. 1000 ml of test liquid weresubsequently introduced. In the experiments shown in Table 1 below, asmall fabric sack containing 2.5 g of an ion exchanger (AMBERJET® UP6040 RESIN mixed bed resin ion exchanger from Rohm+Haas) was suspendedin the liquid, and the examples in Table 2 shown below were carried outwithout the presence of an ion exchanger. The beaker was sealed in anair-tight manner with the glass lid and heated to 88° C., and the liquidwas stirred vigorously using a magnetic stirrer. The electricalconductivity was measured at the beginning of the test and at intervalsof several weeks on a liquid sample taken in advance (LF 530conductivity meter from WTW/Weilheim). After completion of the test, thealuminum samples were assessed visually and, after pickling with aqueouschromic acid/phosphoric acid, evaluated gravimetrically in accordancewith ASTM D 1384-94.

The results are shown in Tables 1 and 2.

TABLE 1 Experiments in the presence of ion exchanger Coolantcomposition: Comparative Example 1: Example 2: Example 3: Example 4:Example 5: Example 60 vol.-% 60 vol.-% 60 vol.-% 60 vol.-% 60 vol.-% MEG(acc. to MEG MEG MEG MEG 40 vol.-% WO 00/17951): 40 vol.-% 40 vol.-% 40vol.-% 40 vol.-% water 0.05% by wt. 60 vol.-% MEG water water waterwater 0.1% by wt. benzotriazole 40 vol.-% 0.1% by wt. 0.1% by wt. 0.1%by wt. hydrogenated 371 ppm by water benzimidazole benzotriazoletolutriazole tolutriazole wt. tetraethoxysilane Electrical conductivity[mS/cm0] Beginning of test: 2.0 4.9 3.3 3.1 1.1 1.9 after 7 days: 2.34.2 1.5 1.5 0.8 1.5 after 35 days: — 7.6 4.1 10.2 — 2.5 after 42 days:36.2 — 3.9 — 3.5 3.3 after 56 days: — — 7.8 — — 5.5 pH Beginning oftest: 6.9 7.5 5.0 5.5 6.6 5.5 End of test: 2.9 6.5 3.8 3.9 4.0 3.7Appearance of slightly tarnished tarnished tarnished tarnished tarnishedaluminum samples tarnished after the test: Weight change [mg/cm²] afterpickling: 1 −0.05 −0.07 −0.06 −0.01 −0.04 −0.03 2 −0.04 −0.06 −0.06−0.01 −0.05 −0.04 3 −0.04 −0.06 −0.06 −0.01 −0.05 −0.02 4 −0.04 −0.06−0.06 −0.01 −0.05 −0.03 5 −0.03 −0.07 −0.06 −0.01 −0.05 −0.03 Mean ofthe samples −0.04 −0.06 −0.06 −0.01 −0.05 −0.03 Solution at end ofyellowish, brownish, colorless, colorless, colorless, colorless, testclear clear clear clear clear clear

In the mixture of monoethylene glycol (=MEG) and water, the volume ratioof 60:40 corresponds to a weight ratio of 62.5:37.5.

In Example 5 according to the invention, the orthosilicate was meteredin so that a silicon content of 50 ppm by weight was present in thecooling liquid.

The results in Table 1 show that a very low electrical conductivity ofless than 4 μS/cm was present even after an uninterrupted experimentduration of 42 days in Examples 2 and 4 in accordance with theinvention, while, with an increase to virtually 40 μS/cm, a significantimpairment had occurred in the coolant with no additives in accordancewith WO 00/17951 (3). Even after an uninterrupted experiment duration of56 days, the electrical conductivity was in some cases stillsignificantly below 8 μS/cm in Examples 2 and 5 in accordance with theinvention.

In no case did significant corrosion on the aluminum samples occur.

TABLE 2 Experiments without ion exchanger Coolant composition: Example1: Example 2: 60 vol.-% 60 vol.-% MEG Example 3: MEG 40 vol.-% water 60vol.-% MEG 40 vol.-% 0.1% by wt. 40 vol.-% water benzotriazole water0.1% by wt. 742. ppm by wt. 0.1% by wt. benzo- tetraethoxy- hydrogenatedZEBRA triazole silane tolutriazole Electrical con- ductivity [μS/cm]Beginning of 3.2 3.2 2.1 test: after 7 days: 5.0 5.6 after 14 days: 5.85.2 5.8 after 28 days: 8.2 6.9 after 35 days: 11.2 6.9 8.6 after 42days: 13.1 7.9 9.3 after 49 days: 16.1 7.6 9.7 after 56 days: — 7.8after 63 days: — 7.1 after 77 days: — 6.6 17.5 pH Beginning of 5.0 5.05.2 test: End of test: 3.6 4.9 3.4 Appearance of almost almost tarnishedaluminum samples unchanged unchanged after the test: Weight change[mg/cm²] after pickling: 1 −0.01 0.00 −0.02 2 0.00 0.00 −0.02 3 0.000.00 −0.04 4 0.00 0.00 −0.04 5 0.00 0.00 −0.04 Mean of the 0.00 0.00−0.03 samples Solution at end colorless, colorless, colorless, of testclear clear clear

In the mixture of monoethylene glycol (=MEG) and water, the volume ratioof 60:40 corresponds to a weight ratio of 62.5:37.5.

In Example 2 according to the invention, the orthosilicate was meteredin such that a silicon content of 100 ppm by weight was present in thecooling liquid.

The results from Table 2 show that a very low electrical conductivity ofsignificantly less than 10 μS/cm was present even after an uninterruptedexperiment duration of 77 days in Example 2 in accordance with theinvention; the electrical conductivity after 77 days was againsignificantly below 20 μS/cm in Example 3 in accordance with theinvention.

In these experiments too, no or no significant corrosion occurred on thealuminum samples.

1. A method of cooling a fuel cell comprising providing in a fuel cellcoolant system an aqueous fuel cell coolant composition having aninitial electrical conductivity of at most 50 μS/cm, wherein said fuelcell coolant composition consists essentially of an alkylene glycol andwater solution, and an additive consisting of one or more five memberedheterocyclic compounds (azole derivatives) having 2 or 3 heteroatomsselected from the group consisting of nitrogen and sulfur, which containno or at most one sulfur atom and which may carry a fused aromatic orsaturated six-membered ring, and an orthosilicate in an amount toprovide a silicon content of from 2 to 2000 ppm by weight.
 2. The methodof claim 1, wherein the coolant composition consists essentially of from0.05 to 5% by weight of the azole derivatives.
 3. The method of claim 1,wherein the azole derivative is benzimidazole, benzotriazole,tolutriazole and/or hydrogenated tolutriazole.
 4. The method of claim 2,wherein the azole derivative is benzimidazole, benzotriazole,tolutriazole and/or hydrogenated tolutriazole.
 5. The method of claim 1,wherein the alkylene glycol is monoethylene glycol.
 6. A method ofincreasing the useful life of a fuel cell coolant composition consistingessentially of a glycol and water solution and having an initialelectrical conductivity of at most 50 μS/cm, which method comprisesadding to the fuel cell coolant composition an additive which consistsessentially of 0.05 to 5% by weight of an azole derivative and anorthosilicate in an amount to provide a silicon content of from 2 to2000 ppm by weight, the additive being present in the fuel cell coolantcomposition in an amount sufficient to impart to the coolant compositiona lesser increase in the electrical conductivity over time in use ascompared to a coolant composition in which the additive is absent.